1. Field of the Invention
The present invention relates to a gas-tube arrester, and more particularly, to a gas-tube arrester, in which line electrodes and an earth electrode are facing each other with insulators interposed, for protecting communication equipment from lightning or other external electrical surges.
2. Related Art
First of all, some examples of gas-tube arresters known in the prior art will be explained with reference to FIGS. 1 to 5.
An arrester 200 shown in FIG. 1 is mounted on a telephone line or any other communication line to protect the communication equipment from lightning or other external electrical surges. For the arrester 200, as shown in FIG. 2, an arrester body 220, in which argon or other inert gas is gas-tightly contained and line electrodes 206 and an earth electrode 208 are arranged with insulators 210 interposed, is often employed.
If a power supply cable line (100, 200, or 6600 Volts AC) is repeatedly in contact with a telephone line, a high voltage is repeatedly applied to the arrester body 220. Then, an electric discharge continues in the arrester body 220. Eventually, the arrester 200 may be overheated and cause a fire or other disaster.
As a precaution for preventing the arrester 200 from overheating, as shown in FIGS. 1 and 2, an insulating film 204 made of polyester or other thermoplastic resin is placed between the arrester body 220 and a conductive metal leaf spring 202 attached on the outer circumferential surface of the arrester body 220. Thus, a fail-safe mechanism is implemented in the arrester 200.
The metal leaf spring 202 is connected through an opening 205 of the insulating film 204 to the earth electrode 208 by a spot-welding or the like.
Using the arrester 200 having this kind of fail-safe mechanism, when a high voltage is applied repeatedly and an electric discharge continues in the arrester body 220, the arrester 200 will then be overheated. Then, the insulating film 204 made of a thermoplastic resin is melted by the heat of the arrester body 220. Then, the conductive leaf spring 202 is urged to the line electrodes 206 and the earth electrode 208 and connects the line electrodes 206 to the earth electrode 208. Therefore, the earth electrode 208 and line electrodes 206 are electrically shorted. This stops the consecutive electric discharge in the arrester body 220. Consequently, a fire resulting from the overheated arrester 200 can be avoided.
However, in the arrester 200 having only such a fail-safe mechanism as shown in FIGS. 1 and 2, if the argon or other inert gas contained in the arrester body 220 leaks for some reason, even if an external surge is applied to the arrester 200, an electric discharge cannot occur in the arrester body 220. This may cause damage to the communication equipment or some unit of a communication system.
To cope with the foregoing drawback, Japanese Unexamined Patent Publication (Kokai) No. 53-52961 (U.S. Pat. No. 4,212,047) has proposed an arrester having both a vent-safe mechanism and a fail-safe mechanism as shown in FIG. 3.
In the arrester, insulating films 204a and 204b clamped between line electrodes 206 and a conductive leaf spring 202 electrically coupled to an earth electrode 208 have respective slits 212 and 212.
Owing to the slits 212, as shown in FIG. 4, a space the thickness of the insulating films 204a and 204b is formed between the conductive leaf spring 202 and the line electrodes 206.
In the gas-tube arrester shown in FIGS. 3 and 4, a fail-safe mechanism and a vent-safe mechanism is implemented.
To be more specific, when an arrester body 220 is heated due to consecutive electric discharge in the arrester body, a fail-safe mechanism operates. That is to say, the insulating films 204b made of a thermoplastic resin melt, and a conductive leaf spring 202 electrically connects the line electrodes 206 to an earth electrode 208 to stop the consecutive electric discharges. This prevents occurrence of a fire.
If argon or other inert gas contained in the arrester body 202 leaks, discharge cannot occur in the arrester body. In this case, if an external surge is applied to the arrester, spaces formed with slits 212 on the tops of the line electrodes 206 (FIG. 4) allow an electric discharge between the line electrodes 206 and the conductive leaf spring 202. Thus, a vent-safe mechanism operates.
However, in the arrester shown in FIGS. 3 and 4, the temperature of the arrester body 220 increases due to consecutive discharge. Even if the insulating films 212 melt to bring the conductive leaf spring 202 into contact with the line electrodes 206 and thus the fail-safe mechanism operates, the contact resistance is still high because the conductive leaf spring 202 and the line electrodes 206 are brought into contact merely with a spring force.
A temperature cycle test was conducted in the range from -40.degree. C. to +60.degree. C. to test the gas-tube arresters of this kind, in which a conductive leaf spring 202 comes into contact with line electrodes 206. However, in some samples of arrester, the contact between the conductive leaf spring 202 and line electrodes 206 could not be attained. As a result, it was found that a fail-safe function, which is based on a permanent contact, cannot be guaranteed.
In an effort to guarantee the fail-safe function, the present inventor has tested the arrester shown in FIG. 5 which has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 53-52960 (U.S. Pat. No. 4,062,054).
In the arrester shown in FIG. 5, insulating films 205a and 205b having heat resistance (insulating film 205b is not shown) are used instead of the insulating films 204a and 204b shown in FIG. 3, and a solder plate 211 for shielding a slit 212 is placed between each of the insulating films 205a and 205b, and a conductive leaf spring 202.
The fail-safe mechanism of the foregoing arrester operates in such a manner that, when the solder plates 211 are melted by heat developed in the arrester 220, the fused solder connects each of line electrodes 206 to the conductive leaf spring 202. This connection is achieved reliably with the fused solder. Therefore, the fail-safe mechanism can make a reliable connection.
However, in the arrester shown in FIG. 5, the soft solder plates 211 are always pressed by the conductive leaf spring 202. Therefore, portions of the solder plates 202 that are shielding the slits 212 warp gradually toward the line electrodes 206. The solder plates 211 and line electrodes 206 may come into contact unexpectedly. Thus, the arrester cannot assure total reliability.